In order to investigate the effect of surface treatment on fatigue properties of martensitic type stainless steel, SUS420J2, rotating bending fatigue tests were carried out using the steels surface-treated by three kinds of conditions. The surface treatments tested were CrN coating, radical nitriding and duplex treatment of CrN coating after the radical nitriding, and the results were discussed in comparison with those of quenched and tempered steel. Fatigue strengths were markedly increased by CrN coating, nitriding and duplex treatment. However, there was no or little effect of CrN coating to nitrided steel on the fatigue strength. Surface fracture yielded in quenched and tempered and CrN coated steels, while fractures occurred from inclusions at the subsurface of specimens in nitrided and duplex treated steels. The growth of an internal crack was suppressed at the boundary of fish eye to the direction to specimen surface by the nitriding in nitrided and duplex treated steels. The ligaments between the fish eye boundary and specimen surface showed a brittle fracture in both steels, especially in duplex treated steel there was a few steps on the ligament meaning the coalescence of cracks initiated from droplets at specimen surface and inclusion in subsurface, independently.
